1. Field of the Invention
The invention relates to a filter for cleaning exhaust gas from diesel engines. The invention is specifically directed to a wall flow filter for removal of NO2, CO, incompletely combusted hydrocarbons and particulate matter in exhaust gas from a diesel engine.
The invention is further directed to use of fuel borne additives being added to diesel fuel prior to combustion and a catalysed wall flow filter arranged in exhaust gas channel of a diesel engine.
The invention has particular use in diesel engine driven cars, ships, trains, vans, lorries and similar, where the above impurities are formed and must be removed in order not to pollute the atmosphere. This removal will be a government requirement in more and more countries, as the pollution is harmful to human beings, animals and even buildings.
2. Description of Related Art
The harmful effect of exhaust gas from diesel engines has been known for a long time and several attempts have been made to solve this problem.
Kawanami et al. disclose in EP patent publication No. 0 888 816 a method for removal of NOx and particulate matters from diesel exhaust gas with a filter. The filter is covered with a layer of catalyst comprising Cu, Pr, Fe, Ce, Co, Ni, La and Nd. The filter catches soot particles which, however, will accumulate. Furthermore, test results show that the filter catalyst performs satisfactory at about 450° C. However, diesel exhaust gas often has a temperature much lower, especially when cars are driven at normal, moderate velocities.
Another catalyst for purification of diesel engine exhaust gas is described by Kim in the US patent application No. 2003/0 104 932, and this catalyst comprises Pt and Pd on a carrier of Zr-W oxide. The carrier is made by preparing a Zr-W containing H2SO4 gel, forming, drying and calcination, which is an elaborate method. The carrier is not especially durable at high temperatures, which occur when carbon from accumulated soot particles is oxidised.
It is further known that certain additives being added to the Diesel fuel prior to combustion reduce emission of particulate matter and unburned hydrocarbons. Those additives typically consist of oil-soluble organo-metallic complexes.
It has now been found that when using an exhaust gas filter being coated with a catalyst comprising various oxides and noble metals it is possible to remove impurities from diesel exhaust gas down to a very low level.
It has further been found that combination of catalytic treatment of exhaust gas from a diesel engine with the above metal oxide/noble metal catalyst composition and fuel born organo-metallic complexes further improve purification of diesel exhaust, in particular combustion of soot being trapped on the catalytic filter at lower temperatures.